Embodiments of the present invention relate to electrochemical ion exchange.
Ion exchange cells are used to remove or replace ions in solutions, for example in the production of high purity water by deionization, in waste water treatment, and in the selective substitution of ions in solution. Ion exchange materials include cation and anion exchange materials that contain replaceable ions or which chemically react with specific ions, to exchange cations or anions, respectively, from a solution stream. A typical conventional ion exchange cell comprises ion exchange resin beads packed into columns and a stream of solution to be treated is passed through the column. Ions in the solution are removed or replaced by the ion exchange material, and product solution or waste emerges from the outlet of the column. When the ion exchange material is-overwhelmed with ions from the solution, the beads are regenerated with a suitable solution. Cation exchange resins are commonly regenerated using acidic solutions or salt brine (eg. for water softeners), and anion exchange resins are most often regenerated with basic solutions or brine.
Electrochemical ion exchange uses a water-splitting ion exchange membrane (also known as a bipolar, double, or laminar membrane) positioned between two facing electrodes with a dielectric spacer between the membranes, as for example, described in commonly assigned U.S. Pat. No. 5,788,826 (Nyberg) which is incorporated herein by reference in its entirety. The water splitting membranes have both a cation exchange layer and an anion exchange layer. When a sufficiently high electric field is applied through the membrane by applying a voltage to the two electrodes, water is irreversibly dissociated or “split” into component ions H+ and OH− at the boundary between the cation and anion exchange layers. The resultant H+ and OH− ions migrate and diffuse through the ion exchange layers in the direction of the electrode having an opposite polarity (eg. H+ migrates to the negative electrode). During the regeneration step, the H+ and OH− ions formed at the membrane interface cause the rejection of cations and anions removed in a previous deionization step, reforming the acid and base forms of the cation and anion exchange materials. Electrical regeneration in this way avoids the use, and subsequent disposal problems, of hazardous chemicals that are used to regenerate conventional ion exchange beads.
The ion exchange membranes of the prior art are typically smooth and flat, and also often have a uniform cross-sectional thickness, to minimize variations in current densities across the membranes in electrochemical cells. Also a separate dielectric spacer, such as a plastic netting material, is typically placed between the membranes to maintain the membranes at a uniform distance from one another to further reduce current density variation and maintain consistent mass transport rates and pressure drops across the gap between the membranes. The membrane thickness is maintained uniform to provide a constant spacing between ion exchange membranes to generate uniform current densities across membrane surfaces. Various methods can be used to maintain a uniform thickness on the membrane. The membrane should also be flat and smooth to facilitate the backwashing of ion exchange resins in electrodeionization devices, and well as the periodic replacement of these resins. Membranes have a smooth flat surface also reduces the pressure drop between adjacent membranes. However, conventional ion exchange membranes generally provide slower ion exchange rates and consequently, slower solution treatment flow rates and outputs, than conventional ion exchange bead systems. Consequently, the membrane systems have limited utility due to space volume versus solution treatment throughout considerations.
Furthermore, the dielectric spacer layers that are used to separate the membranes in the electrochemical cells have to be sufficiently thick to be free-standing and structurally sound to withstand handling when inserted between the membranes in the fabrication of a cell. Typically, the spacer layers are fabricated from polymer, such as polyethylene or polypropylene, and can have a regular pattern. A typical thickness of the spacer layer is from about 0.25 to about 1 mm (10 to 40 mils). Spacer layers thinner than 0.25 mm are difficult to handle and can result in stretching of spacer strands, tearing or creasing, in the assembly of cells, and they are also generally more expensive and difficult to make. A further problem in attempting to reduce spacer thickness arises because the pressure of the solution passed through the cell needs to be increased to deliver the same solution flow rate. Thus, generally, relatively thick spacer layers are used in the electrochemical cell, further increasing the bulk volume of the cell and reducing solution treatment output per unit volume of cell space.
The stack of membranes and spacers is also difficult to assemble into or remove from the cell for cleaning or replacement. Typically, a number of membranes and spacers are used in a cell, and it is desirable to be able to more easily handle the stack of membranes. The membrane stack can also become easily disoriented in the cell during assembly of the cell or during use. Also, when membranes are sealed into water-tight electrochemical cells, it is difficult to open the cells to remove the membranes without damaging the cell or membranes. A cell structure that can be easily assembled or opened is desirable.
Thus, it is desirable to have an electrochemical ion exchange cell capable of providing good ion exchange rates. It is also desirable to have a water-splitting membrane and spacer that exhibits sufficient strength for handling and use and which is not excessively thick. It is further desirable to limit the number of steps needed to manufacture such cells, reduce the number of parts for assembly, and reduce assembly errors. It is also desirable to have a cell that can be easily assembled or disassembled without damaging or disorienting the membranes.